Aircraft use a variety of actuators to move flaps or other flight control surfaces such as horizontal stabilizers, to vary the aerodynamics of the aircraft and control the aircraft during flight, takeoff, and landing. The actuators are often subjected to strong loads from aerodynamic forces and it is possible for the joint that connects the actuator to the main airframe to fail and the actuator (and possibly the flight control surface) to become separated from the aircraft. Another failure possibility is rupture of the screw shaft which could lead to separation of the parts. These would be considered “catastrophic events” from a safety standpoint according to the EASA CS25 certification standard for large commercial airplanes.
Consequently, many actuators have redundant connections to the airframe that are engaged if the primary load path of the actuator fails. One such connection is to a failsafe bar within a screw actuator. A failsafe bar may be attached to an airframe at one end and extend within a screw shaft of an actuator. The failsafe bar may be coupled to the screw shaft in such a way that the failsafe bar is not loaded during normal loading of the actuator, but becomes loaded and provides part of a secondary load path if the actuator's primary connection to the airframe fails. Backlash (play) may be provided at the ball joint level, for example, between the failsafe bar and its associated secondary lug (hereafter referred to as the “attachment part”) fixed to the aircraft structure, for accommodating primary load path deflection in normal functioning conditions whilst preventing loading of the secondary load path.
It is now mandatory according to airworthiness regulations (e.g. EASA CS25) to have a means of detecting failure of the primary load path.